feat(native-spans)!: change buffer foundation#2046
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## yannham/span-vec-fields-integration #2046 +/- ##
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Add the supporting types for the change-buffer module behind a `change-buffer` feature flag: ChangeBuffer (raw memory wrapper), OpCode/BufferedOperation, SpanHeader (repr(C)), SmallTraceMap, Trace, and error types. These are the building blocks for ChangeBufferState which follows in the next commit. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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| //! | ||
| //! The change buffer is currently designed and used for dd-trace-js, but the idea could be extended | ||
| //! to other runtime where the FFI cost is high. | ||
| #[allow(unused)] |
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With the change-buffer feature enabled, this outer attribute only applies to the following ChangeBufferError enum, so the rest of the new module still emits unused-import and dead-code warnings. I checked RUSTFLAGS='-D warnings' cargo check -p libdd-trace-utils --features change-buffer, and it fails on the unused imports/helpers and BufferedOperation; the required all-features clippy invocation also promotes these warnings to errors. Make this an inner module attribute (or remove the unused items) so the feature can pass CI.
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ekump
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Jun 1, 2026
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| /// A handle to a change buffer shared with another runtime. The memory is shared, meaning that | ||
| /// cloning is cheap (copying the pointer and length). | ||
| #[derive(Clone, Copy)] |
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Would it be better for this to be only Clone to prevent any accidental implicit copying?
Actually, does this need to even be clone?
ekump
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Jun 2, 2026
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Is this file supposed to be here? It doesn't look like it's used for anything.
ekump
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Jun 2, 2026
| "serialization", | ||
| ] } | ||
| indexmap = "2.11" | ||
| rustc-hash = "2" |
ekump
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Jun 2, 2026
| .get(*index..*index + size) | ||
| .ok_or(ChangeBufferError::ReadOutOfBounds { | ||
| offset: *index, | ||
| len: self.len, |
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minor: could len for this this error be slightly confusing? Is it reasonable for someone reading this error to assume len means the length of bytes attempting to be read? self.len is the length of the entire buffer.
ekump
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Jun 2, 2026
| // construction time. We do not materialize other references during the lifetime of `slice`. | ||
| let slice = unsafe { self.as_mut_slice() }; | ||
| let target = slice | ||
| .get_mut(offset..offset + 4) |
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This is doing unchecked usize adds, which I think is normally fine. Is there any chance that we could get a corrupted or malicious offset value upstream that would trigger an overflow here?
ekump
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Jun 2, 2026
| pub service_id: u32, // offset 56 | ||
| pub resource_id: u32, // offset 60 | ||
| pub type_id: u32, // offset 64 | ||
| /// Index into ChangeBufferState.spans for meta/metrics overflow data. |
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I may just be confused, but is this doc comment accurate?
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Jun 2, 2026
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| //! In order to amortize the cost of crossing the FFI when using native spans, the runtime write | ||
| //! events in the change buffer instead many times and only flush it by batch, where the call to | ||
| //! libdatadog happens. Libdatadog processes the change buffer and reconstruct the corresponding | ||
| //! spans. |
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minor grammar tweak:
Suggested change
| //! In order to amortize the cost of crossing the FFI when using native spans, the runtime write | |
| //! events in the change buffer instead many times and only flush it by batch, where the call to | |
| //! libdatadog happens. Libdatadog processes the change buffer and reconstruct the corresponding | |
| //! spans. | |
| //! In order to amortize the cost of crossing the FFI when using native spans, the runtime writes | |
| //! events into the change buffer instead of calling libdatadog many times, and only flushes by | |
| //! batch — that flush is where the call to libdatadog happens. Libdatadog then processes the change | |
| //! buffer and reconstructs the corresponding spans. |
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What does this PR do?
Add foundation types for dd-trace-js change buffer: buffer, opcodes and trace. Follow up of #2043.
Motivation
See #2022. Required for the integration of native spans into dd-trace-js.
Additional Notes
This PR just introduces the types and doesn't use them yet, hence the
allow(unused).How to test the change?
Unit tests provided.